git-gui: Cleanup handling of the default encoding.

- Make diffs and blame default to the system (locale)
  encoding instead of hard-coding UTF-8.
- Add a gui.encoding option to allow overriding it.
- gitattributes still have the final word.

The rationale for this is Windows support:

1) Windows people are accustomed to using legacy encodings
   for text files. For many of them defaulting to utf-8
   will be counter-intuitive.
2) Windows doesn't support utf-8 locales, and switching
   the system encoding is a real pain. Thus the option.

This patch also adds proper encoding conversion to Apply Hunk/Line.

Signed-off-by: Alexander Gavrilov <angavrilov@gmail.com>
Tested-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

lib/encoding.tcl

@@ -274,3 +274,17 @@ proc tcl_encoding {enc} {
     }
     return {}
 }
+
+proc get_path_encoding {path} {
+	set tcl_enc [tcl_encoding [get_config gui.encoding]]
+	if {$tcl_enc eq {}} {
+		set tcl_enc [encoding system]
+	}
+	if {$path ne {}} {
+		set enc2 [tcl_encoding [gitattr $path encoding $tcl_enc]]
+		if {$enc2 ne {}} {
+			set tcl_enc $enc2
+		}
+	}
+	return $tcl_enc
+}